Gas-cluster-jet nozzles are employed as a means of generating a neutral beam of gas-clusters for use in, for example, molecular beam epitaxy and gas-cluster ion-beam formation.
Gas-cluster-jets are typically formed by ejecting a high-pressure (typically about 2 atmospheres or more) condensable source gas into a vacuum through a nozzle. Various nozzle forms have been employed, including conical, sonic, and Laval forms. In each case, as the high-pressure gas expands into the vacuum through the nozzle, adiabatic expansion occurs and the source gas at least partially condenses into a beam of gas-clusters. The clusters may range in size of from as few as 2 to as many as tens of thousands of molecules (atoms in the case of monatomic gases) loosely bound together into clusters. In general the gas-cluster-jet contains a wide distribution of gas-cluster-sizes. Additionally, a large quantity of un-clustered gas atoms/molecules may also flow into the vacuum through the nozzle.
Many practical applications of gas-cluster-jets are best implemented in a low-pressure vacuum (as are the cluster generation, ionization, and acceleration processes), so it is important to be able to remove un-clustered gas from the vacuum system continuously and efficiently, so as to maintain the integrity of the vacuum in the system generating and employing the gas-cluster-jet. Conventionally, this has been done by the use of skimmers and collimators to separate the gas-cluster-jet from the un-clustered gas, by the use of differential vacuum pumping techniques, and by brute force application of large vacuum pumps with high pumping speed (typically, all three techniques employed in combination).
A field of application for gas-cluster-jets that has emerged as a practical industrial process in recent years has been in the formation of a gas-cluster ion-beam (GCIB). When a gas-cluster-jet is ionized using a conventional ionization process such as electron impact ionization, a fraction of the gas-clusters become ionized and can be accelerated and otherwise manipulated by electric and magnetic fields and may thus be employed in various useful industrial and scientific applications.
Gas-cluster ion-beams have been used to process surfaces for purposes of cleaning, etching, smoothing, film growth, doping, infusion, and the like. Gas-cluster ions are ionized, loosely bound, aggregates of materials that are normally gaseous under conditions of standard temperature and pressure, typically consisting of from a few hundred atoms or molecules to as many as a few ten thousands of atoms or molecules. Gas-cluster ions can be accelerated by electric fields to considerable energies of tens of thousands of eV or even more. However, because of the large number of atoms or molecules in each gas-cluster ion, and because of the loose binding of the clusters, their effect upon striking a surface is very shallow—the cluster is disrupted at impact and each atom or molecule carries only a few eV of energy. At the surface, instantaneous temperatures and pressures can be very high at gas-cluster ion impact sites, and a variety of surface chemistry, etching, shallow infusion, and cleaning effects can occur. Gas-cluster ion-beams have been used to clean and smooth medical implants and to adhere drugs to the surfaces of medical devices including stents (See U.S. Pat. No. 7,105,199 granted Sep. 12, 2006 to Blinn et al. and U.S. Pat. No. 6,676,989, granted Jan. 13, 2004 to Kirkpatrick et al.)
Other applications of GCIB include numerous uses in the field of electronics, including film formation, surface etching, surface smoothing, surface modification, shallow doping, and production of strained semiconductor materials.
Numerous prior art patents have disclosed details of GCIB apparatus, including the means of forming the neutral gas-cluster-jet. As examples see U.S. Pat. No. 5,814,194, Deguchi et al.; see JP 25093312A2, Toshihisa et al.; see U.S. Pat. No. 6,486,478, Libby et al.; see US 2006/0118731A1, Saito et al.; and see US 2003/0109092A1, Choi et al. All have employed the concepts: nozzle, skimmer, differential vacuum pumping, and large vacuum pumps.
Therefore it is an object of this invention to provide methods and systems for improved generation of a gas-cluster-jet by employing improved vacuum chamber geometry.
Another object of this invention to provide a GCIB processing system employing and benefiting from methods and systems for improved generation of a gas-cluster-jet with improved vacuum chamber geometry.